The present invention concerns the use of blade cyclic pitch control in horizontal axis wind energy converters.
The term blade cyclic pitch variation is used in the technology of helicopters and other rotorcraft when the blade pitch angle changes periodically with the period of rotor revolution. Control of blade pitch cyclically as the rotor rotates on its substantially vertical axis generally removes aerodynamic pitching and rolling moments and protects the rotorcraft in maneuvering flight from high gyroscopic moments. Hence, rotorcraft control systems superimpose cyclic pitch control on their collective pitch systems, which generally control the lift of the machine.
With wind turbines, which are usually horizontal axis machines and whose technology has been considered non-analogous, blades are sometimes feathered with increased wind speeds. This is in effect collective pitch control. The principles of cyclic pitch control have not been applied to such machines; although vertical axis gyromills use in effect a highly complex form of cyclic pitch variation.
It is general practice in the design of horizontal axis wind turbines to use blade feathering in order to prevent rotor overspeeding or overtorquing beyond rated wind speed. The blade feathering mechanism is complex and costly; the propeller type blades are exposed to high dynamic loads, and the propeller rotor can only be yawed very slowly to avoid blade overstressing from gyroscopic effects. Some early wind turbines had hinged or teetering blades in combination with the blade feathering mechanism. For large wind turbines this configuration has been again considered; relief from the weight and cost of the blade feathering mechanism has been sought by feathering only parts of the blades.
It has long been recognized that overspeed or overtorque control beyond rated wind speed might be accomplished by so gearing the rotor to the mast as to yaw it out of the wind, thus avoiding the complex and costly blade feathering mechanism. However, in addition to severe resultant loads on the mast, the blades suffer severe stresses when operating headed angularly out of the wind and when subjected to side wind gusts. To avoid destruction of the rotor, the permissible yaw rate must be so limited that effective response to such gusts is impossible.
In some simple fixed pitch horizontal axis wind mills whose forward-presented rotors are not geared to the mast, the hub axis has been positioned offset from the vertical axis about which yawing is permitted, and the trailing vane has been pivoted, spring loaded against one stop, and angularly movable at high winds to a second stop. The offset thrust of the rotor is roughly balanced by the moment of the wind force on the vane, in its position against the first stop, until the wind becomes too high for safe operation. The spring on the pivoted vane then yields; the vane moves to the other stop and yaws the rotor angularly out of the wind, the generator meantime being electrically disconnected. Spring loaded vanes have also been used to position rotors at angles advantageous for starting. No such system has been used as a matter of normal operation to maintain the rotor at a design operating parameter through a range of wind speeds.
In the present invention, as hereinafter described, blade cyclic pitch feedback is utilized somewhat in the manner of rotorcraft technology, and high yaw rates to accommodate shifting winds become possible with effective control of the yaw angle of the rotor axis relative to wind direction.